Augustine

United States

59.363°N, 153.43°W; summit elev. 1252 m

All times are local (unless otherwise noted)

During 8-14 February, seismicity at Augustine remained at low levels, but was still above background. Also, low-level ash plumes and occasional pyroclastic flows occurred. Satellite and seismic data, and other remote observations indicated that a lava dome had been growing at the volcano's summit since 28 January. On the evening of 7 February, gas, ash, and incandescent blocks were observed descending the upper NE flank of Augustine. All available data indicated that as the lava dome grows, it periodically becomes unstable and small portions of it avalanche down the N flank, producing pyroclastic flows. The number of these flows gradually decreased during several days before 10 February, suggesting that the rate of lava extrusion also slowly declined. Data from continuous GPS receivers on the island indicated that the flanks of the volcano began to deflate around 28 January, marking a reversal in the swelling trend observed beginning in roughly June of 2005. AVO reported that based on all available data, eruptive activity associated with lava-dome building will continue over the next few days or weeks and may continue intermittently over the next several months. Augustine remained at Concern Color Code Orange.

Geologic Background. Augustine volcano, rising above Kamishak Bay in the southern Cook Inlet about 290 km SW of Anchorage, is the most active volcano of the eastern Aleutian arc. It consists of a complex of overlapping summit lava domes surrounded by an apron of volcaniclastic debris that descends to the sea on all sides. Few lava flows are exposed; the flanks consist mainly of debris-avalanche and pyroclastic-flow deposits formed by repeated collapse and regrowth of the volcano's summit. The latest episode of edifice collapse occurred during Augustine's largest historical eruption in 1883; subsequent dome growth has restored the volcano to a height comparable to that prior to 1883. The oldest dated volcanic rocks on Augustine are more than 40,000 years old. At least 11 large debris avalanches have reached the sea during the past 1800-2000 years, and five major pumiceous tephras have been erupted during this interval. Historical eruptions have typically consisted of explosive activity with emplacement of pumiceous pyroclastic-flow deposits followed by lava dome extrusion with associated block-and-ash flows.